The invention relates to a method and apparatus for introducing a particulate solid sample for analysis, and particularly for flame or plasma spectroscopy.
A number of problems have impeded the development of practical methods for flame or plasma analysis of solid samples. The sample must be introduced uniformly to the plasma to minimize variations in plasma characteristics, such as temperature, available energy, etc. Uniform sample delivery is required for most commercially available instrumentation for wavelength scans, background corrections, and integration times of sufficient duration to maximize sensitivity. Sample flow must be controlled to avoid overloading the analyzing device. Sufficient mass must be analysed to avoid problems related to inhomogeneity.
Several approaches for delivering solid samples for direct analysis by flame or plasma have been proposed. The direct introduction of powders into a flame or inductively coupled plasma by a fluidized-bed approach has also been proposed.
In the prior system the same injector gas entrains the sample, and delivers it to the plasma. The gas flow rate is chosen to be compatible with the requirements of the analyzing device, rather than a flow which would result in an optimum fluidized bed. With the prior system the sample flow is difficult to control and cannot be maintained uniform over a period of time. Because of this, quantitative measurements can usually be done only by introducing the total (weighed) sample. Also, with the prior system, samples cannot be changed without disrupting the plasma.
An object of the present invention is to allow continuous, uniform, and controlled delivery of particles for flame or plasma analysis.
Another object is to allow independent control of sample introduction, or sample change without disrupting the flame or plasma.
It has been found that the above objectives can be met by providing two stages of flow comprising a first stage of relatively low flow for entraining the fluidized solid particles in controlled amounts and a second stage of relatively high uniform flow as required by the analyzing device.
The present invention provides an apparatus for introducing a particulate sample to an analyzing device comprising: an enclosure having an inlet for a first carrier gas, and an outlet for gas entrained sample particles; a sample container disposed within the enclosure, said container having an opening for receiving the first carrier gas, supplied at a relatively low flow rate to the enclosure; agitating means for fluidizing a sample contained by the sample container; conduit means communicating with the interior of the sample container, said conduit defining the outlet from the enclosure of a sample entrained by carrier gas supplied to the container; and a combining chamber connected to said conduit, said combining chamber including an inlet for receiving a second carrier gas, of relatively high flow rate, and an outlet for connection to an analyzing device.
The present invention also provides a method for introducing a particulate sample to an analyzing device comprising: fluidizing the sample by agitation; passing a first carrier gas of relatively low substantially uniform flow rate through the fluidized sample for entrainment of a portion of the sample; and combining the first carrier gas and entrained sample with a second carrier gas having a relatively high substantially uniform flow rate so that the combined flow has a high substantially uniform flow rate that is compatible with the requirements of the analyzing device.